A car starts from rest and travels for 5.1 s with a uniform acceleration of +1.8 m/s2. The driver then applies the brakes, causing a uniform acceleration of -2.1 m/s2. The breaks are applied for 2.00 s.
(a) How fast is the car going at the end of the braking period?
(b) How far has the car gone from its start?

Using these equations, which ones should I use for a? for b?
vf = at + vi
x = 1/2(vf + vi)t
x = 1/2at2 + vit
vf2 = vi2 + 2ax

Question

A car starts from rest and travels for 5.1 s with a uniform acceleration of +1.8 m/s2. The driver then applies the brakes, causing a uniform acceleration of -2.1 m/s2. The breaks are applied for 2.00 s. 
(a) How fast is the car going at the end of the braking period?
(b) How far has the car gone from its start?

Using these equations, which ones should I use for a? for b?
vf = at + vi 
x = 1/2(vf + vi)t 
x = 1/2at2 + vit 
vf2 = vi2 + 2ax